This invention relates generally to a motor control system and it also relates generally to individual components of the system. More particularly, but not by way of limitation, the invention relates to a transformer, a unitary housing, and a double interlock mechanism, all of which are suitable for use in an apparatus for operating a three-phase motor to drive a submersible pump. The present invention is also more particularly directed to a system and method for controlling the energization of a transformer's secondary winding circuit to which a three-phase motor is connected.
Submersible pumps are used, for example, in oil wells at remote locations. Three-phase electric motors are typically used to drive these pumps. Such a motor is rated for a nominal line-to-voltage which must be provided within a specified tolerance for the motor to work. This voltage is typically provided from an electric utility through a transformer and motor controller to provide the suitable voltage and control to operate the motor as desired.
Transformers and motor controllers which have been used in the past have been separate products. That is, the transformer has had its own housing and the motor controller has had its own housing. External connections between the two are needed to have the two work together. Although having two separate units might allow more flexibility in choosing components for a particular application, it has the possible shortcomings of increased price for two rather than one unit and of increased costs for shipping and warehousing. Two separate units would also likely require more space at the location where they are to be used. Therefore, there is the need for a unitary power supply package wherein a transformer and motor controller are interconnected and housed in a single compact unit which can be readily transported to remote locations and easily connected to a source of electricity and a load, such as a three-phase electric motor driving a submersible pump.
To facilitate the use of such a unitary power supply package at a remote location, it should be designed so that a human operator on the ground can have access to at least some internal parts should they need to be repaired or checked in the field. Ground-level access should also be provided so that the operator can readily select a desired output suitable for the load to be energized and readily control a master on/off switch of the power supply package. Access to at least high voltage components should, however, be prevented by automatic interlocks which operate when the master switch is "on."
For safety and economy, the transformer within the power supply package should be designed to provide all needed output and operating voltages and it should also be designed to shield against electrostatically and magnetically coupled transients. Appropriate switching and fusing regardless of the desired output should also be provided.
Such a unitary power supply package should also include a readily transportable housing which accommodates all the other needs mentioned above.
Another feature of the prior transformer and motor controller systems is that the motor controller package includes an air-insulated master power switch, a combined current limiting and load sensing fuse and an electrically operated start-stop contactor switch mechanism connected in series. This places all these components on one side of the transformer.
The disadvantage of the typical master power switch is that it is expensive. It is expensive because it must be constructed to operate safely within its air-insulated environment. Further, the master power switch is typically not used as a complete safety disconnect because it is not constructed to disconnect safely when the motor is energized; rather it is used to isolate the downstream components after the contactor switch mechanism has disconnected the motor. An air-insulated power switch adequate to break the load directly would be even more expensive.
A typical combined current limiting and load sensing fuse used in prior motor controllers is also relatively expensive; therefore, everytime a short-circuit fault or other current overload condition clears the fuse, it must be replaced with a similarly expensive fuse. Another disadvantage of using the fuse in the prior manner is that it cannot be rated for all the transformer outputs which might be available.
In view of these additional disadvantages, there is the need for a system which incorporates a relatively inexpensive, truly emergency safety master power switch which is directly manually operable without the aid of any tools to break a fully loaded circuit. There is also the need for the system to utilize fusing which is relatively inexpensive and which is fully effective to protect the system upstream of a short-circuit fault regardless of a selected transformer output.